The present invention relates to radiation dosimetry and specifically to systems which employ ionization chambers, and that are suitable for the precise dosage measurement of low energy radiation emitted by diagnostic and therapeutic mammography and other medical equipment.
It is well recognized in the medical community that the use of x-ray mammography is an important tool in the early diagnosis and treatment of breast cancer and similar conditions. Many physicians recommend yearly mammography screening for women in high risk groups. However, in recent years, a concern arose that the radiation levels emitted by mammography equipment during screening or therapy constituted a health hazard. Accordingly, mammography equipment was redesigned for use at lower radiation dose levels, in the range of 30 KVp to 50 KVp, as compared to 60 KVp to 150 KVp typical radiation energy levels in diagnostic radiology.
Coincident with the reduction in radiation dose levels in mammography was the need for dosimetry systems capable of measuring and calibrating the equipment used at these radiation levels to insure that safe levels were not exceeded and to accurately determine therapeutic doses. Thus, dosimetry equipment usable in mammography needed the capability of accurately and consistently measuring low energy radiation doses, as would be received by the breast.
The prior art recognized that small ionization chambers, contained in a breast phantom or other simulating device and used in conjunction with a charge accumulating/integrating-type dosimeter, could be used in this application. Typically, the prior art ionization chambers would include a biased charge collecting device mounted in a small box or cylinder, with a radio-transparent window exposed to the radiation source. Most such prior art chambers employed aluminized Mylar.RTM. brand plastic film as a window material, the goal being minimal attenuation of low energy radiation. Although aluminized Mylar.RTM. brand plastic film windows provided good low energy response, that response varied as a function of the energy level of the radiation. Polycarbonate was also used as a window material.
Unfortunately, the prior art ionization chambers do not provide a flat response. Therefore, at the low energy levels used in mammography, mathematical corrections must be used by the operator in order to provide the plus or minus 5% accuracy mandated by state and federal law. Also, aluminized Mylar.RTM. brand plastic film windows, being only one to two mils thick, are extremely fragile and must be stretched to be attached to the chamber body. Further, if it is desired to take dose measurements in water, such as in a linear accelerator chamber used in electron beam therapy, a special protective cover for the fragile window must be used.
What is needed, then, is a dosimetry system which incorporates an ionization chamber that provides accurate and flat response, without the use of external mathematical correction factors, at low radiation energy levels and that includes an ionization chamber with a radiation window less susceptible to mechanical damage.